Normal analog TV video signal has a vertical sync pulse with serrations to help keeping the horizontal oscillations in sync. Also, as I understand it, in case of an interlaced video, these serrations have double frequency to account for half-period shift between the video fields.

So it seems that the horizontal oscillator has something to ignore that second serration and to avoid syncing to the doubled frequency.

But what happens if the serrations are sent at triple frequency (for non-interlaced scan)?

I understand that different TVs may react differently, but my hope is that the number of different horizontal sync approaches is very small.

For a little background:

Yes, it's weird to have a triple frequency pulses, but that's what one computer from 80s outputs.

Each line of its video signal is 2/3 of image and 1/3 of blanking plus horizontal sync pulse. The circuit for the last 1/3 just goes in cycles, but suppressed during the image. When the vertical sync pulse comes (3 lines), that 1/3 gets inverted and not suppressed, so it happens thrice on each line.

I guess they couldn't make it at double frequency, because they didn't have enough logic elements in the chip. And I guess they couldn't simply invert the black lines, because too long pulses aren't allowed (?).

  • \$\begingroup\$ That is pretty hypothetical, but most likely nothing happens. They are not valid hsync pulses, and to continue interlacing the vsync must start at exactly half line offset. And to preserve average DC balance the triple frequency equalization pulses need to be shorter in time, just like the standard equalization pulses are half the duration of standard hsync pulse. Why would there be need to send triple frequency pulses in any occasion? Non-interlaced will work with standard serrations. \$\endgroup\$
    – Justme
    Commented Apr 28, 2022 at 11:28
  • \$\begingroup\$ @Justme what are standard serrations? \$\endgroup\$ Commented Apr 28, 2022 at 13:55
  • \$\begingroup\$ The standard double-frequency serrations you already talked about. And the double-frequency serrations are not strictly needed for interlacing, you can send interlaced signal without serrations if you like. \$\endgroup\$
    – Justme
    Commented Apr 28, 2022 at 13:57

1 Answer 1


Regarding just the sweep oscillators, ignoring effects on the video signal itself:

Depending on how far back you go -- the standard method AFAIK is a blocking oscillator. Roughly speaking, this is a type of relaxation oscillator, which stays latched (on/off) until its timing element passes a threshold, then it toggles state.

If we superimpose ("inject") a signal on top of that timing signal (or the threshold, equivalently), we can make it trigger sooner, and in sync with the source.

More concretely: suppose there is an oscillator, with a voltage ramp signal of say 0 to 5V. If we superimpose a 0.5V sync signal on it, then for injected frequencies within 0.5/5 = 10% of normal (actually, -10 to 0%, since the injected signal can only make it fire sooner than usual), it will lock 1:1. Or within 20% of 2x, it will lock 2:1, etc. Note that it won't lock 9:1 vs. 10:1, at least not consistently: the difference in threshold between the 9th and 10th pulses will not be well defined, and it'll probably lock to the earlier one instead.

(This is all very hand-waved, mind. Driven oscillators are, in general, chaotic systems, so technically just about anything is possible, given just the right circuit. The usual circuits, adjusted properly, I don't think are too pathological, so this should do for basics.)

Exactly what thresholds and ratios are used, likely vary between TV sets. Also likely they're defined by component tolerances, so, 5 and 10% resistors and capacitors being common back in the day, they probably chose a similar injection level. That is, NTSC itself (or whatever else) might be very precisely defined, but the time constants in the set cannot be made so tight (with economical manufacture), so they set it on the loose side so it needs less adjustment. (The HOLD controls adjust the oscillator's natural frequency, so this will even be trimmed by the user.)

I don't know that other methods were used much if at all, e.g. a fully fleshed-out PLL system? Or like a one-shot timer (ramp starts after sync pulse), which might technically do given a valid video signal, but since all the tube voltages were derived from the horizontal sweep system, at least horizontal had to keep running (and for that matter, vertical had to keep running to prevent burn-in of a line on the tube, so the same goes for that).

Digital sets (90s+?) likely measure the timing directly, and whether they accept or reject pulses between expected intervals, I don't know. Of course a multisync (computer) monitor might well try and lock to it (1:1); or just complain that it's an illegal mode.

So, a 3:1 ratio probably is fine with most analog sets. It's really weird putting in extra sync pulses though ("blacker than black" level during a sweep?).

  • \$\begingroup\$ Is it possible to know for sure the type of the sync method just by looking at the TV circuit diagram? \$\endgroup\$ Commented Apr 29, 2022 at 6:56
  • \$\begingroup\$ @DmitriUrbanowicz If it's a discrete circuit and you're familiar with analog circuit analysis/behavior, or IC based and you can find the datasheets. Oh, something I missed: note that which column out of 3 the set syncs to, won't be well defined, and it may jump around from frame to frame in the presence of interference, etc. \$\endgroup\$ Commented Apr 29, 2022 at 16:52

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